Paper EM-MoA8
Encapsulation of AlGaN/GaN High Electron Mobility Transistor based Hydrogen Sensor for Humid Ambient Sensing Application

Monday, October 30, 2017, 4:00 pm, Room 14

Hydrogen is environmentally friendly alternative energy source and carrier for automotive and fuel cell applications as well as in many industrial processes. Hydrogen gas is colorless, odorless, extremely reactive with oxygen, and has very low ignition energy. Especially, leaking gas from pressurized container may elevate its temperature, and induce spontaneous flammable ignition due to its negative Joule-Thomson coefficient. Therefore, hydrogen gas sensing systems are essential in various hydrogen related applications. GaN based material system is well-suited to hydrogen sensing because of its wide bandgap for high temperature operation, and mechanical and chemical robustness for device reliability. Many types of devices based on the GaN, including Schottky diodes, metal oxide semiconductor diodes, GaN nanowires and AlGaN/GaN high electron mobility transistors have been developed for fast and sensitive detection of hydrogen. Among them, AlGaN/GaN HEMTs with a 2 dimensional electron gas (2DEG) channel induced by piezoelectric and spontaneous polarization between the AlGaN and GaN layers showed high sensitivity to change in surface charge created by catalytic reaction of Pt or Pd with hydrogen. With 30 % Al concentration in AlGaN layer, 5~10 times higher electron densities in 2DEG are obtained compared to typical GaAs or InP HEMTs, which induces higher current and better sensitivity of the device. However, one of issues with semiconductor based hydrogen sensors is the fact that their sensitivity is significantly degraded in the presence of humidity or water. Water molecules block the catalytically active sites of sensing material, and results in the significant reduction in hydrogen detection signal. By employing encapsulation layer which prevents water molecules from adsorbing on the active sites of the sensor while selectively allowing penetration of hydrogen molecules, this issue can be solved. In this paper, we demonstrate that the device encapsulated with a moisture barrier does not suffer from any significant change in hydrogen detection sensitivity in the presence of moisture and that the devices can be repeatedly cycled to temperatures up to 300°C without any change in characteristics. Also, the device did not respond to the other gases including CH₄, CO, NO₂, CO₂, and O₂.